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⚡Voltage Drop (NFPA 72) | NSVCad | YouTubeToText
YouTube Transcript: ⚡Voltage Drop (NFPA 72)
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Video Summary
Summary
Core Theme
Electrical resistance in conductors causes voltage drop, which is a critical factor in fire alarm system design to ensure reliable operation, especially during voltage fluctuations.
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Hello everyone and welcome to this
video. All electrical conductors include
a small amount of resistance. This
resistance increases if the length of
the conductor increases or the conductor
size decreases. Think of blowing air
through a hose. If the hose diameter
decreases and or the length increases,
it would be harder to blow through. You
can also think of freeway traffic as
resistance. The freeway is the
conductor. The wider the freeway, the
faster and smoother you travel. As
electrical current flows through the
conductor, it experiences a decrease in
voltage between the source, the starting
point, and various points along the
conductor. Another example to look at is
the voltage drop in a 1,00 ft run of 16
AWG wire would be greater than that of a
1,00 ft run of 12 AWG. This is simply
because a 16 AWG conductor is smaller in
diameter than a 12 American wire gauge
conductor. Fire alarm equipment listed
to the standards of the National Fire
Protection Association and Underwriters
Laboratories UL is tested to determine
if can operate properly at 85% of the
rated name plate voltage. This limit was
set in place to make sure the circuit
can deal with a brownout condition or a
possible voltage drop which might result
from excessive resistance in the system
wiring. Signaling line circuit voltage
drop calculation confirms that the
minimum required voltage reaches the
last device on the power loop even under
worst case load conditions as required
in the NFPA72.
Fire alarm designers are required to
prepare voltage drop calculations for
the notification appliance circuits. As
part of the design, you as a designer
can use several different methods to
calculate voltage drop on a fire alarm
circuit. One method calculates the
overall voltage drop and the other
calculates the actual voltage drop for
each length of cable and device within
the circuit. These voltage drop
calculations must be included in the
submittal plans and specifications. The
suggested maximum allowable voltage drop
on a fire alarm circuit is 10% or the
voltage drop included in the fire alarm
control panel installation guide,
>> Lump sum method. Step one, take the
total current of the circuit. You can
achieve this figure by adding up the
current draw of each device on the
circuit. This will represent a in the
formula. Step number two, measure out
the length of the circuit in feet. Do
not double the distance of the circuit
for two wire loops unless you want to
use a multiplying factor of 10.8 versus
21.6. 6 C step number three this will
represent L in the formula step number
three use a multiplying factor of 21.6
six. This number represents the
resistivity of copper conductors. This
is a constant used in the formula. Step
number four. Find the circular mills for
the particular gauge wire you are using.
This can be found in the National
Electrical Code NEC chapter 9 table 8.
14 AWG= 4,110
12 AWG= 6530.
This will represent CM in the formula.
This formula calculates the total
voltage drop VD by multiplying the total
current A by the circuit length L and
the copper resistivity constant 21.6
then dividing by the conductor's size in
circular mills CM. This example uses the
lump sum method to confirm that a 0.356
amp circuit running 450 ft of 12 AWG
wire will result in a 0.530
volt drop. This final step shows you how
to convert the voltage drop into a
percentage by dividing the lost voltage 0.530
0.530
volt by the circuit starting voltage
which is 24 volts and multiplying by
100. confirming this circuit has a safe
2.2% loss. Now remember, you can also
perform this calculation for each
individual segment of wire and each
device on the circuit. This is known as
the pointto-point method. It is a more
accurate approach because it allows you
to break down the circuit and identify
exactly where the circuit should end due
to voltage drop limitations. [music]
Next, let's explore the NSV cabling app,
a specialized tool for faster, more
accurate circuit design and voltage drop
calculations. Start the process by
clicking alarm cabling command. A
configuration form pops up, allowing you
to define the key circuit parameters.
Within this form, you gain full control
over the circuit setup. You can define
the circuit type such as loop, zone,
sounder, and power. Select your
preferred unit of measurement like feet
or meters and precisely define circuit
tags and all electrical parameters in
Cabling begins from the fire alarm
control panel. From there you will
connect all the devices in accordance
with the circuit class, the project
design and the site specific
requirements. As you connect each
device, the software automatically
assigns an address based on the device
tag you selected in the draw cable form.
In the lower left corner of the screen,
you can monitor live data, including
cable length, current, and voltage drop,
giving you instant feedback as you
design. The NSVC CAD cabling app uses
the point-to-point method for voltage
drop calculations. The main advantage of
the point-to-point method is that it
provides accurate segment by segment
voltage drop verification, ensuring that
every device in the circuit receives
adequate operating voltage, something
the lump sum method cannot reliably guarantee.
guarantee.
After finalizing your design, generate
the riser diagram by typing NHRD in the
command bar, then clicking on the fire
alarm control panel to open the riser
form. Configure the block count and
device spacing in the dialogue and then
With the NSV cabling app, you can move
beyond guesswork to achieve faster, more
accurate, and fully compliant circuit
designs. For detailed guides and product
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